The invention relates to a novel apatite-coated metallic material having improved surface quality and biocompatibility, a process for its preparation, and the use of the material for bone implants, in particular dental implants, artificial joints and fixative material for accident surgery (osteosynthesis material).
It is known that coated implants integrate better with calcium phosphate, in particular with the bone mineral hydroxyapatite (HAP (Ca10(PO4)6OH)). Different processes are used for coating implants with calcium phosphate, such as, for example, plasma injection, sol-gel processes, electrophoresis, and electrochemically assisted deposition.
Electrochemically assisted deposition has advantages compared with other processes on account of the possibility of the production of uniform (even with very rough surfaces) and thin layers, the possibility of the specific control of the phases to be deposited by means of electrical parameters, and produces lower costs in preparation.
In electrochemically assisted deposition, calcium phosphate precipitates on the cathode, which forms the implant. The resulting layer is very porous and can therefore be removed easily from the surface. There are various possibilities for decreasing the porosity.
According to U.S. Pat. No. 3,892,648, an emulsion of bone powder and collagen is applied electrochemically to the metallic implant and a stronger adhesion is achieved therewith by means of the collagen.
In DE 19504386 A1, the deposited calcium phosphate layer is combined in graded form with the metal surface. Calcium phosphate crystals are surrounded by a growing oxide layer. A disadvantage of this process is: it functions only with implants of titanium or titanium alloys. The porosity of the coating is not lowered and the mechanical properties are thus not improved.
In the patent U.S. Pat. No. 5,458,863, using an electrochemically assisted deposition, a brushite layer is first produced, which is then converted to hydroxyapatite at temperatures between 20 and 100xc2x0 C. The adhesion between layer and substrate is improved by regular removal of gas bubbles on the substrate surface during the coating. It is disadvantageous in this process that the conversion process lasts about 36 hours. At temperatures of 750xc2x0 C., according to WO 9813539, hydroxyapatite crystals are formed from the electrochemically assisted deposition of a calcium phosphate phase and the adhesion is improved. Here, the higher temperatures are especially to be mentioned as particularly disadvantageous. In the patent U.S. Pat. No. 5,205,921, after the electrolytic deposition the adhesion of the layer produced is improved by means of ultrasonic processes in a methanol bath. The method is based on the fact that crystallites having low adhesion to the substrate are detached again by the influence of ultrasound.
There is still a great need for implant materials having an improved surface and compatibility with the biological system.
An object of the invention is therefore an apatite-coated metallic material having decreased porosity and improved adhesion. Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
According to the invention, these objects are achieved by means of an apatite-coated metallic material, in which the coating consists of a thick covering of hydroxyapatite crystals preferably having a needle length (see, e.g., Racquel Z. LeGeros, Calcium Phosphates in Oral Biology and Medicine (1991) p. 20) in the range from 200 to 300 nm and/or amorphous calcium phosphate spheres preferably having a diameter in the range from 35 to 200 nm with the total coating having a layer thickness preferably of  greater than 1 xcexcm particularly from 2 to 5 xcexcm, and the coating has a specific surface area of less than 15 m2/g.
Examples of the metallic material to be coated include titanium or titanium alloys, CoCrMo alloys or stainless steels.
According to the invention, the novel apatite-coating material is dissolved by means of an electrochemically assisted process using a substrate electrode formed from the metallic material and a counterelectrode in which, as electrolyte, an aqueous solution containing calcium and phosphate ions is used.
According to the invention, the coating is carried out by cathodic polarization in a number of successive process cycles. A process cycle consists of cathodic polarization in one or more successive steps with identical or different high constant current densities, and a rinsing and/or drying phase following thereon.
The concentration ratio of calcium and phosphate ions in the electrolyte preferably corresponds substantially to that of hydroxyapatite.
By means of one embodiment of the process according to the invention, a decrease in the porosity takes place in that the process is repeated two or more times in a number of cycles with electrochemical calcium phosphate deposition and subsequent rinsing and/or drying.
Electrochemically, hydroxyapatite (HAP) or its precursors (amorphous calcium phosphate (ACP)/mixed states of ACP/HAP) are deposited on the metallic material. The needle length size of the hydroxyapatite crystals is preferably between 200 and 300 nm. The amorphous spheres can be varied in their diameter, preferably in the range from 35 to 200 nm. The compressed layers are preferably achieved by an exchange between short coating phases and rinsing and/or drying phases following thereon. The drying is carried out at room temperature. During the drying, the used electrolyte liquid is stripped off the porous layers. On the next immersion, the cavities fill with fresh electrolyte liquid. An electrochemically assisted deposition of calcium phosphate phases thus also takes place in the cavities. Moreover, the metallic material body is preferably moved continuously during the coating and drying phases in order to obtain a uniform coating, even with specially shaped material bodies and very rough/porous surfaces.
In one embodiment of the process according to invention, the cathodic polarization takes place at a constant current density of 0.5 mA/cm2 to 20 mA/cm2 or in the individual process cycles at different current densities, the current density being decreased in the subsequent cycles.
The invention also comprises the use of the novel apatite-coated metallic materials for the production of implants, in particular dental and joint implants, and material for the stabilization of the bone in fractures (osteosynthesis material).
The entire disclosure[s] of all applications, patents and publications, cited above or below, and of corresponding German Application No. 10128259.1, filed Jun. 6, 2001, is hereby incorporated by reference.